Thank you for all your posts, ladies and gents, this did indeed help! I got a Charliecard at the Downtown Crossing Charliecard store and used it every day. I was in Boston for four days and only spent $18.55 on the card. I still have it, with $1.45 on, but as it doesn't expire for a decade, it may get used again.
Also handy that I could use it on the Silver Line buses running down Washington Street near the Hampton Inn Crosstown, my hotel - there were no train routes nearby, but thankfully the Silver Line was just a couple of blocks away.
When I went into the Charliecard store I was immediately told to go outside and queue outside the door... they evidently don't like queues inside it!